Process for neutralizing bauxite dissolution residual substance and process for producing aluminum hydroxide

ABSTRACT

A process for neutralization is provided which can maintain a high solid content of a bauxite dissolution residual substance slurry even after a neutralization treatment is completed and the stabilize pH within a short time of the neutralization treatment. Such a process for neutralizing a bauxite dissolution residual substance containing a desiliconizing product which is generated in the production process of aluminum hydroxide using Bayer process, includes: mixing a bauxite dissolution residual substance or a bauxite dissolution residual substance slurry with sulfuric acid in an amount ranging from 0.6 to 1.2 equivalence to the total amount of sodium contained in the bauxite dissolution residual substance or a bauxite dissolution residual substance slurry, such that the solid content after mixing becomes 400 to 700 g/l to gelate the mixture, and thereafter obtaining a bauxite dissolution residual substance neutralized slurry.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Divisional Application of U.S. application Ser. No. 12/810,452filed Jun. 24, 2010 issued as U.S. Pat. No. 8,679,427, which is aNational Stage Entry of PCT International Application No.PCT/JP2008/073903 filed Dec. 22, 2008, which claims benefit of JapanesePatent Application No. 2007-335140 filed Dec. 26, 2007. The above-notedapplications are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a process for neutralizing a bauxitedissolution residual substance containing a desiliconizing product whichis generated in the production process of aluminum hydroxide using aBayer process, in particular a process for neutralizing a bauxitedissolution residual substance being capable of maintaining pH stablyafter neutralization, and a process for producing aluminum hydroxideusing the process.

Priority is claimed on Japanese Patent Application No. 2007-335140,filed Dec. 26, 2007, the content of which is incorporated herein byreference.

BACKGROUND ART

In a Bayer process for producing aluminum hydroxide from bauxite as araw material, approximately 860 kg of a bauxite dissolution residualsubstance is generated per 1 t of reduced mass of alumina as a product.Since such a large amount of a bauxite dissolution residual substance isgenerated, it is necessary to treat a bauxite dissolution residualsubstance quickly in order to produce aluminum hydroxide effectively.

In a Bayer process, a soluble alumina component contained in bauxite oreis usually dissolved and extracted at a high temperature with analkaline solution such as a caustic soda or a sodium aluminate solution,and the resultant extracted slurry is subjected to solid-liquidseparation into a bauxite dissolution residual substance and a sodiumaluminate solution.

Since the separated bauxite dissolution residual substance exhibitsstrong alkali, it is subjected to treatment such as pH adjustment so asto be suitable for a final disposal method (sea dumping, an inning). Thetreated bauxite dissolution residual substance is usually exhausted in aslurry state outside the system.

Bauxite dissolution residual substance separated from sodium aluminatesolution exhibits a pH of approximately 12-13, even after it is washed.In general, a bauxite dissolution residual substance is subjected toneutralizing treatment by adding mineral acid such as, for example, asulfuric acid, hydrochloric acid is further added to the bauxitedissolution residual substance slurry.

However, a bauxite dissolution residual substance slurry has theproperty that the pH value of the slurry increases with the passage oftime after neutralization. This is due to the fact that theneutralization decomposition rate of a sodalite contained in a bauxitedissolution residual substance is low, or to the fact that elution ofalkaline components drawn into pores of sodalite crystal, such as sodiumaluminate, etc.

For example, even if a bauxite dissolution residual substance slurry isneutralized so that the pH value thereof will be approximately 7,isolation and elution of Na₂O components from non-reacted sodalitecontained in the bauxite dissolution residual substance gradually occur,thereby increasing the pH value of the bauxite dissolution residualsubstance slurry. Thus, in the neutralization process of a bauxitedissolution residual substance slurry with acid, neutralization isperformed aiming at a pH which is lower than desired pH value.

However, a bauxite dissolution residual substance slurry has a propertythat the bauxite dissolution residual substance slurry will be viscousto gelate in neutralizing, when the pH becomes approximately 4.5.Therefore it is not possible to aim at too much low pH value.

In addition, the amount of Na₂O contained as a sodalight in a bauxitedissolution residual substance slurry varies greatly, depending on theraw material bauxite and operating conditions. For example, the quantityof sodalight often varies greatly depending on the amount of reactivesilica contained in the bauxite as a raw material. In other words, it isnot always possible to control the pH value so that it is within adesired range by simply adjusting the pH value, due to the sodalitecontained in the bauxite dissolution residual substance slurry.

In order to solve this problem, the following Patent document 1discloses a method for neutralizing a bauxite dissolution residualsubstance characterized in that the neutralizing treatment is dividedinto at least three steps and that the first step of neutralization isperformed such that objective value of pH value is aimed at a valueranging from 7 to 10. In the Patent Document 1, it is disclosed that inaccordance with this method, solidification of a bauxite dissolutionresidual substance slurry can be avoided, considering of material of theneutralization equipment becomes unnecessary, and the total amount ofsulfuric acid required for neutralization can be greatly reduced,compared to those of the method for neutralizing comprising one step andthe method for neutralizing comprising two steps. However, this methodrequires a long time because neutralization is performed at pluralsteps.

As a solution for solving this problem, the following Patent Document 2discloses a method for neutralizing the bauxite dissolution residualsubstance slurry which comprises: a single step of the neutralizingbauxite dissolution residual substance slurry using a thin acid of 3N orless so as not to lower the pH value thereof, thereby preventingincreases of viscosity and gelation.

In accordance with this method, the operation time for adding acid isshortened. However, as for neutralization and decomposition of asodalite contained in the bauxite dissolution residual substance, thedecomposition rate of Na₂O contained in sodalite using a thin acidbecomes low. Thus, it also takes a long time for the pH to becomestable. In addition, in this method, the quantity of the treatmentsolution increases because the use of thin acid, a neutralizing reactionapparatus becomes large, thereby increases the cost of equipment. Inaddition, it requires concentration of the treated liquid, if handlingsuch as the transportation of slurry after processing is considered.

-   [Patent Document 1] Japanese Patent Publication No. 59-15719-   [Patent Document 2] Japanese Patent No. 3,663,647

DISCLOSURE OF THE INVENTION

In view of such circumstances, the inventors of the present inventionhave thoroughly researched performing neutralization which can maintainhigh solid content of bauxite dissolution residual substance slurry evenafter the neutralization treatment is completed and quickly stabilize pHwithin a short time after neutralization treatment is completed. As theresult, they found that the above problem can be solved by addingsulfuric acid in an amount approximately equivalent to the amount ofhigh concentration bauxite dissolution residual substance slurry so asto gelate the slurry, thereby re-slurrying bauxite dissolution residualsubstance slurry to complete the present invention.

In order to achieve the above object, the present invention has adoptedthe following constitution.

A first aspect of the present invention provides a process forneutralizing a bauxite dissolution residual substance containing thedesiliconizing product which is generated in the production process ofaluminum hydroxide using a Bayer process, including: mixing a bauxitedissolution residual substance or a bauxite dissolution residualsubstance slurry with a sulfuric acid in an amount ranging from 0.6 to1.2 equivalence to the total amount of sodium contained in the bauxitedissolution residual substance or bauxite dissolution residual substanceslurry, such that the solid content after mixing becomes 400 to 700 g/lto gelate the mixture, and thereafter obtaining bauxite dissolutionresidual substance neutralized slurry.

A second aspect of the present invention provides the process forneutralizing a bauxite dissolution residual substance in accordance withthe first aspect, in which the bauxite dissolution residual substance isa slurry obtained by washing or diluting bauxite dissolution residualsubstance containing a desiliconizing product which is generated in theproduction process of aluminum hydroxide using a Bayer process

A third aspect of the present invention provides the process forneutralizing a bauxite dissolution residual substance in accordance withthe first aspect or the second aspect, in which the sulfuric acid is aconcentrated sulfuric acid or a fuming sulfuric acid.

A fourth aspect of the present invention provides the process forneutralizing a bauxite dissolution residual substance in accordance withany one of the first to the third aspects, in which the mixing isperformed within 5 minutes.

A fifth aspect of the present invention provides the process forneutralizing a bauxite dissolution residual substance in accordance withany one of the first to the fourth aspects, further including the stepof agitating the gelated mixture.

A sixth aspect of the present invention provides a process for producingan aluminum hydroxide by a Bayer process, including: neutralizing abauxite dissolution residual substance containing a desiliconizingproduct which is generated in the process using the process forneutralizing a bauxite dissolution residual substance in accordance withany one of the first aspect to fifth aspect.

A seventh aspect of the present invention provides a process forproducing an aluminum hydroxide in accordance with the sixth aspect, inwhich the neutralized bauxite dissolution residual substance isdischarged outside the system within 1 to 8 hours after neutralization.

According to the present invention, there is little increase in of theamount of neutralized solution and pH of the slurry after neutralizationis stabilized quickly in neutralizing bauxite dissolution residualsubstance containing desiliconizing product, and hence it is possible todetermine early whether or not pH can be controlled to be desired value.As a result, it is possible to treat a bauxite dissolution residualsubstance at a low cost of transport and a low throughput, with hardlydetaining bauxite dissolution residual substance neutralized slurryduring production of aluminum hydroxide.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph which shows relationship between the pH (difference topH of 96 hours later) of a bauxite dissolution residual substanceneutralized the slurry of Examples 1 to 3 and Comparative Examples 1 to3, and the passage of time after neutralization.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be given below, with referenceto the drawings. According to the present invention, a bauxitedissolution residual substance or a bauxite dissolution residualsubstance slurry containing a desiliconizing product to be generatedduring a production process of aluminum hydroxide by a Bayer process ismixed with sulfuric acid so as to obtain the predetermined amount ofsolid content needed, and then the resultant mixture is gelated. Forexample, this gel mixture converts to a slurry again when it is allowedto stand, thereby obtaining a bauxite dissolution residual substanceneutralized slurry with little pH change. According to the presentinvention, a bauxite dissolution residual substance or a bauxitedissolution residual substance slurry is neutralized by such anoperation.

It should be noted that, in the following explanation, a gel mixturethat has been neutralized with sulfuric acid is referred to as a mixtureand a bauxite dissolution residual substance neutralized slurry may besimply referred to as neutralized slurry.

(Bauxite Dissolution Residual Substance, Bauxite Dissolution ResidualSubstance Slurry)

In the production process of aluminum hydroxide by a Bayer process, abauxite dissolution residual substance which is separated from a sodiumaluminate solution by solid liquid separating means such assedimentation separation or filtration is usually in the form of a cakeor slurry having a high solid content just after separated, but thebauxite dissolution residual substance may be converted to a slurry bydiluting it with, for example, water, if necessary, taking easiness ofthe later handling into consideration. In the present specification, adissolution residual substance after solid-liquid separation is referredto as bauxite dissolution residual substance, and what is in the form ofslurry obtained by diluting or washing mentioned later a bauxitedissolution residual substance is simply referred to as a bauxitedissolution residual substance slurry. Both of them are those which canbe neutralized in the neutralization process of the present invention.It should be noted that the difference between a cake and slurry is thedifference in solid content. If necessary, the solid content may beadjusted to be 400 to 700 g/l, and preferably 450 to 650 g/l, by mixingit with sulfuric acid, mentioned later.

When the solid content after mixing is less than 400 g/l, the pH afterneutralization is hard to be stabilized. In addition, the amount oftreatment solution increases.

On the other hand, when the amount of solid content is high, neutralizedslurry becomes highly viscous and is hard to be handled. The amount ofsolid content after mixing where it is easy to be handled is not morethan 700 g/l.

The solid content of the bauxite dissolution residual substance or thebauxite dissolution residual substance slurry can be adjusted byconcentrating (for example, by sedimentation) or diluting (for example,dilution with water) the bauxite dissolution residual substance or thebauxite dissolution residual substance slurry.

The step of concentrating or diluting may be a step of washing bauxitedissolution residual substance to recover adhered sodium aluminate. Forexample, a valuable sodium aluminate which adheres to a dissolutionresidual substance can be recovered by performing decantation in acounterflow washing method in a multistage washing thickener.

When a bauxite dissolution residual substance is washed, the resultantdissolution residual substance after washing becomes a slurry containingthin sodium aluminate solution. The solid content of the slurry at thistime is adjusted to be the above range. Since the use of the washedbauxite dissolution residual substance slurry can reduce the amount ofsulfuric acid used in the subsequent step, it is preferable.

(Mixing with Sulfuric Acid)

Neutralization is performed by mixing sulfuric acid in an amount rangingfrom 0.6 to 1.2 equivalent, preferably from 0.7 to 1 equivalent to thetotal Na contained in the resultant bauxite dissolution residualsubstance or bauxite dissolution residual substance slurry, with thebauxite dissolution residual substance or bauxite dissolution residualsubstance slurry.

If the amount of sulfuric acid is less than 0.6 equivalent, then the pHof the mixture after neutralization is likely to be unstable. Inaddition, the resultant mixture may not gelate. Whereas, if the amountof sulfuric acid is more than 1.2 equivalent, then the resultant mixturemay remain a gelatin, and may hardly become slurry again. Theconcentration of sulfuric acid can be adjusted such that the neutralizedslurry has a solid content within the above range.

As for a bauxite dissolution residual substance slurry, the lower thesolid content thereof is, the lower the viscosity thereof becomes, andthe easier handling becomes. Therefore, the concentration of sulfuricacid to be added is preferably as high as possible, so as not to dilutethe bauxite dissolution residual substance slurry. That is, the sulfuricacid to be added to a bauxite dissolution residual substance slurry ispreferably concentrated sulfuric acid, more preferably fuming sulfuricacid, and particularly preferably fuming sulfuric acid of not less than40N. It should be noted that, concentrated sulfuric acid indicates asulfuric acid of not less than 36N.

In addition, as for a bauxite dissolution residual substance, theconcentration of sulfuric acid to be mixed is adjusted such that thesolid content of the neutralized slurry after neutralization is withinthe above range.

When sulfuric acid is mixed with a bauxite dissolution residualsubstance or bauxite dissolution residual substance slurry, the sulfuricacid is added at a considerably high rate, such that pH deteriorates andthe slurry after mixing are gel. Although speed at which a sulfuric acidis added varies depending on the container and agitating method, it maybe aimed at that the whole mixed slurry gelates. It is usually the rateat which addition can be completed in five minutes.

Generally, gelating of a bauxite dissolution residual substance or of abauxite dissolution residual substance slurry should be avoided, becausethe gelation causes clogging of piping and high loading to an agitator.However, if it is allowed to stand as it is, then the gel mixtureobtained by neutralizing under the above conditions will become a slurryagain, and the pH of this slurry will stabilize in a short amount oftime. Thus a bauxite dissolution residual substance neutralizationslurry having a stabled pH is obtained.

(Agitation of Gel Mixture)

The resultant gel mixture is preferably agitated for a short amount oftime. In this case, an agitator applicable to high viscosity substanceis necessary, which is preferable for mixing sulfuric acid easilyuniformly and for shortening the amount of time needed to convert themixture back into the slurry and stabilize the pH.

(pH Stability)

According to the present invention, the difference in pH between each ofthe mixture or neutralized slurry of one hour later, three hours later,and eight hours later from the neutralization and neutralized slurry of96 hours later from the neutralization is usually 1.2, 1.0 and not morethan 0.5, respectively. In particular, when the gel mixture is agitated,the difference in pH becomes 1.0, 0.5 and not more than 0.2,respectively, and hence agitating is preferable. It should be noted thatthe time from the neutralization is one from completing of mixing ofsulfuric acid.

That is, according to the production process of aluminum hydroxide by aBayer process including the neutralization method of bauxite dissolutionresidual substance of the present invention, the gel mixture becomes aslurry and the pH is stabilized within a very short time of 1-8 hoursfrom the neutralization, and hence bauxite dissolution residualsubstance neutralized slurry can be discharged outside the system with apH accurately in the range of the above range of difference in pH.

EXAMPLE

The present invention will be explained specifically referring toembodiments below, but the scope of the present invention is not limitedthereto.

It should be noted that the measurement method in the following examplesand comparative example will be explained.

-   1) pH measurement: A glass electrode-type pH meter was used.-   2) Total soda content of bauxite dissolution residual substance    slurry:-   A sample of the bauxite dissolution residual substance slurry was    placed in an evaporation dish and was dried at 110° C. for 3 hours    to be solidified, the resultant dried solidified sample was crushed,    and thereafter the resultant sample was subjected to fluorescence    X-ray analysis to obtain total soda content (T-Na₂O) contained in    the bauxite dissolution residual substance neutralized slurry.-   3) Solid content:-   The slurry (or gel mixture) was filtered under reduced pressure    using a filter paper, washed with water, and thereafter the    resultant dried sample was measured to obtain the dry mass thereof,    and then the solid content of the slurry was obtained therefrom.

Example 1

A bauxite dissolution residual substance containing a slurry-likedesiliconizing product separated from a sodium aluminate solution duringproduction of aluminum hydroxide by a Bayer process was diluted withwater to obtain a bauxite dissolution residual substance slurry havingsolid content of 500 g/l. This slurry had a composition of 15% by massof SiO₂, 23% by mass of Al₂O₃, 38% by mass of Fe₂O₃, 7% by mass of TiO₂,9% by mass of Na₂O (each of analysis values is expressed, under theassumption that the dry mass of the slurry is 100% by mass.) Inaddition, the pH of this slurry was 12.3.

1.5 kg of the resultant bauxite dissolution residual substance slurrywas put into a 2.5 liter beaker, while the slurry was stirred with athrowing-in type stirrer, 49N fuming sulfuric acid in an amount of 1.0equivalent to the total soda content contained in the bauxitedissolution residual substance was added thereto for approximately 1minute to neutralize the slurry. The solid content in the slurry afteradding the sulfuric acid was 481 g/l.

At this time, the resultant mixture after neutralization became a gel.However, agitating was continued further, and then the resultant mixturebecame a slurry again 25 minutes after the neutralization to obtain aneutralized slurry.

In addition, during agitation, the pH of the slurry was measured bytimewise. The pH of the slurry 1 hour, 3 hours, 8 hours, and 96 hoursafter the neutralization of sulfuric acid was 6.95, 7.33, 7.55 and 7.65,respectively. Changes in the pH were shown in FIG. 1, and tables 1 and2. The pH increased by only 0.1 between 8 hours and 96 hours after theneutralization of sulfuric acid.

Example 2

Treatment and measurement were conducted in the same way as in Example1, with the exception of changing the amount of sulfuric acid to be 0.8equivalent. When sulfuric acid was added, the slurry became a gelmixture. However, as a result of the agitation being continued, theresultant gel mixture became a slurry again 20 minutes after theneutralization to obtain a neutralized slurry. The solid content of theslurry after the addition of sulfuric acid thereinto was 485 g/l.

The pH of the slurry 1 hours, 3 hours, 8 hours, and 96 hours afterneutralization of sulfuric acid was 7.16, 7.97, 8.19 and 8.07respectively. Changes in the pH are shown in FIG. 1 and tables 1 and 2.The pH decreased by 0.12 between 8 hours and 96 hours after theneutralization.

Example 3

Treatment and measurement were conducted in the same way as in Example1, with the exception of changing the amount of sulfuric acid to be 0.7equivalent and not diluting the bauxite dissolution residual substancewith water. When sulfuric acid was added, the slurry became a gelmixture. However, as a result of the agitation being continued, theresultant gel mixture became a slurry again 30 minutes after theneutralization to obtain a neutralized slurry. The solid content of theslurry after adding sulfuric acid thereinto was 645 g/l.

The pH of the slurry 1 hours, 3 hours, 8 hours, and 96 hours afterneutralization of sulfuric acid was 8.71, 8.76, 8.95 and 8.88respectively. Changes in the pH are shown in FIG. 1 and tables 1 and 2.The pH decreased by 0.07 between 8 hours and 96 hours after theneutralization.

Comparative Example 1

Treatment and measurement were conducted in the same way as in Example1, with the exception of changing the amount of sulfuric acid to be 0.5equivalent. When sulfuric acid was added, the slurry remained in aslurry state. The solid content of the slurry after the addition ofsulfuric acid thereinto was 490 g/l.

The pH of the slurry 1 hours, 3 hours, 8 hours, and 96 hours afterneutralization of sulfuric acid was 7.78, 8.46, 9.34 and 10.01respectively. Changes in the pH are shown in FIG. 1 and tables 1 and 2.The pH increased by 0.67 between 8 hours and 96 hours after theneutralization.

Comparative Example 2

Treatment and measurement were conducted in the same way as in Example1, with the exception of using diluted sulfuric acid of 7N as thesulfuric acid. When sulfuric acid was added, the slurry remained in aslurry state. The solid content of the slurry after the addition ofsulfuric acid thereinto was 391 g/l. Each of pH at 1 hours later, 3hours later, 8 hours layer, and 96 hours later from the neutralizationwas 6.18, 6.52, 7.66 and 8.11 respectively. Changes in the pH are shownin FIG. 1 and tables 1 and 2. The pH increased by 0.45 between 8 hoursand 96 hours after the neutralization.

Comparative Example 3

Treatment and measurement were conducted in the same way as in Example1, with the exception of using diluted sulfuric acid of 7N as thesulfuric acid and changing the amount of sulfuric acid to be 0.5equivalent. When sulfuric acid was added, the slurry remained in aslurry state. The solid content of the slurry after the addition ofsulfuric acid thereinto was 432 g/l. The pH of the slurry 1 hours, 3hours, 8 hours, and 96 hours after neutralization of sulfuric acid was7.46, 8.78, 9.47 and 10.39 respectively. Changes in the pH are shown inFIG. 1 and tables 1 and 2. The pH increased by 0.92 between 8 hours and96 hours after the neutralization.

It should be noted that Table 1 is a list showing correlation between pHof bauxite dissolution residual substance neutralized slurry of Examples1 to 3 and Comparative Examples 1 to 3 and passage of time from theneutralization. In addition, Table 2 shows a list of relative value(difference) of pH between 96 hours later and 0 to 8 hours later, andpassage of time from the neutralization, which is a list correspondingto FIG. 1.

TABLE 1 Comparative Comparative Comparative Example 1 Example 2 Example3 Example 1 Example 2 Example 3 Equivalence of sulfuric acid Time(hours) 1.0 0.8 0.7 0.5 1.0 0.5 Before 12.34 12.22 12.39 12.39 12.3412.22 addition 0.25 5.44 6.08 6.45 7.46 6.06 7.33 0.5 5.96 6.76 7.217.67 6.09 7.36 1 6.95 7.16 8.71 7.78 6.18 7.46 3 7.33 7.97 8.76 8.466.52 8.78 8 7.55 8.19 8.95 9.34 7.66 9.47 96 7.65 8.07 8.88 10.01 8.1110.39

TABLE 2 Passage of time after neutralization Comparative ComparativeComparative (hours) Example 1 Example 2 Example 3 Example 1 Example 2Example 3 0.25 2.19 1.99 2.43 2.55 2.05 3.06 0.5 1.69 1.31 1.67 2.342.02 3.03 1 0.7 0.91 0.17 2.23 1.93 2.93 3 0.32 0.1 0.12 1.55 1.59 1.618 0.1 −0.12 −0.07 0.67 0.45 0.92 96 0 0 0 0 0 0

As shown above, in Examples 1 to 3, a gel mixture was generated afterthe neutralization, however, the gel mixture changed into slurry afterbeing left to stand for 20 to 30 minutes. The pH of the neutralizedslurry was approximately stabilized 8 hours after more than 8 hours fromthe neutralization, and the absolute value of range of change in pHbetween 8 hours and 96 hours after the neutralization became not morethan 0.2.

On the other hand, in Comparative Examples 1 to 3, the resultant mixtureremained in slurry state after the neutralization, without gelation.This is because the concentration or the number of equivalent ofsulfuric acid was short. In addition, the pH of the neutralized slurrywas unstable only after 8 hours after the neutralization. That is, theabsolute value of change width of pH 8 hours to 96 hours after theneutralization ranged from 0.45 to 0.92.

The present invention is applicable to a process for neutralizing abauxite dissolution residual substance containing a desiliconizingproduct which is generated in the production process of aluminumhydroxide using Bayer process, in particular a process for neutralizingbauxite dissolution residual substance being capable of maintaining a pHstably after neutralization, and a process for producing aluminumhydroxide using the process.

The invention claimed is:
 1. A process for producing aluminum hydroxideby a Bayer process, comprising: neutralizing a bauxite dissolutionresidual substance containing a desiliconizing product which isgenerated in the production process of aluminum hydroxide using a Bayerprocess, said neutralizing comprises: mixing a bauxite dissolutionresidual substance slurry with sulfuric acid in an amount ranging 0.6 to1 equivalence to the total amount of sodium contained in the bauxitedissolution residual substance slurry, such that the solid content aftermixing becomes 400 to 700 g/l to gelate the mixture, and thereafterobtaining bauxite dissolution residual substance neutralized slurry,wherein the sulfuric acid has a concentration of 40N or greater.
 2. Theprocess for producing aluminum hydroxide as claimed in claim 1, whereinthe bauxite dissolution residual substance slurry is obtained by washingor diluting a bauxite dissolution residual substance containing thedesiliconizing product.
 3. The process for producing aluminum hydroxideas claimed in claim 1, wherein the sulfuric acid is a concentratedsulfuric acid or a fuming sulfuric acid.
 4. The process for producingaluminum hydroxide as claimed in claim 1, wherein the mixing iscompleted in 5 minutes or less.
 5. The process for producing aluminumhydroxide as claimed in claim 1, further comprising a step of agitatingthe gelated mixture.
 6. The process for producing aluminum hydroxide asclaimed in claim 1, wherein said neutralizing is carried out in aneutralization system, and said bauxite dissolution residual substanceneutralized slurry is discharged outside the neutralization systemwithin 1 to 8 hours after the neutralization.